The synthesis reaction of the compound having the general formula (I):
through oxidation of a compound having the general formula (II) has been described in a variety of documents:

For example, patent EP 0295117 B1 shows a synthesis method using a 3-chlorobenzoic derivative.
Such synthesis method has evident disadvantages both in terms of yield and of costs, the latter related to the impossibility of re-using the oxidising agent.
A method alternative to the previous one was proposed in document WO 01/030760 A1, where the oxidation step is conducted in the presence of trifluoroperacetic acid (TFPA), obtained from trifluoroacetic acid (TFA) in the presence of hydrogen peroxide and boric acid.
However, also this method is unsatisfactory given the following drawbacks: first of all the trifluoroacetic acid is an extremely expensive reagent which, as a consequence, negatively affects the sales price of the product therewith obtained.
Moreover, during the reaction, hydrogen fluoride is released which eats into the vitreous coatings used in industrial reactors despite operating at temperatures close to ambient temperature. To this purpose, document WO 01/030760 A1 suggests using a corrosion inhibitor which nonetheless entails an additional expense to the overall cost of the process.
Moreover, the use of a corrosion inhibitor would not be adequate to protect all the equipment needed for the recovery process of TFA from the corrosive effect of hydrogen fluoride. The TFA recovery and subsequent reutilisation is a necessary operation dictated by the high cost of TFA compared to common oxidants.
The drawbacks of WO 01/030760 A1 have been overcome thanks to the teaching of document WO 2007/122440 A1 where, instead of TFPA, oxidation is conducted in the presence of trichloroperacetic acid (TCPA).
According to the description, TCPA is the effective oxidising species and is formed in situ by the reaction of an oxidising agent with trichloroacetic acid (TCA).
As well as acting as an oxygen acceptor, TCA should also conveniently act as a reaction solvent.
However, at the temperature at which the reaction takes place, TCA is solid (melting point=54-58° C.) so that, for its use as a reaction solvent, a second solvent needs to be added to the TCA to lower its melting point to a temperature compatible with the reaction temperature.
The solvents suitable for this purpose are, among others, dichloroacetic (DCA) and monochloroacetic (MCA) acids.
In particular, a mixture composed of TCA (70-80%) and DCA (30-20%), characterised by a melting point of 15° C.-30° C., has been shown as suitable for conducting such oxidation reaction where, as mentioned above, the sole purpose of the DCA is to depress TCA's melting point.
However, also this method has the drawback that the oxidant species TCA only allows to operate in a temperature range such as to favour the formation of a by-product having the general formula (III), described below, which has a twofold disadvantage.
First of all, the reaction forming the by-product consumes useful product to the detriment of the yield.
In addition, the by-product having the general formula (III) is difficult to be separated from the compound having the general formula (I) given its low solubility in common organic solvents thereby requiring an expensive purification process adding to the cost.
A further disadvantageous aspect is that the oxidant species TCA can only be used at temperatures compatible with the oxidation reaction of the compound having the general formula (II) to the compound having the general formula (I) if the reaction is conducted in the presence of a species acting as a solvent both for the reagent having the general formula (II) and for the oxidant TCA itself, making both the recovery operation of the product having the general formula (I) and the recovery of the oxidant TCA more complicated.
In addition, it is realistic to believe that, in the teaching of the prior art document WO 2007/122440 A1, the dichloroacetic acid does not transform into dichloroperacetic acid by means of the hydrogen peroxide or other oxidant, because the species TCA, present in significant molar excess of the oxidant and more reactive towards the oxidants, captures all the available oxygen.
The present invention therefore sets out to provide a new method for the preparation of the compound having the general formula (I) using an economically advantageous oxidation method convenient to implement in industrial applications.